A Republican takeover of the Senate would be a slap down for Democrats, and most political handicappers consider that scenario unlikely. But Republicans are expected to add to the 41 seats they have now, eating into the Democrats' almost-filibuster-proof margin and, perhaps, forcing them to approach lawmaking in a more bipartisan way than has recently been the case. (Of course, there's also the possibility of gridlock.)

Remember, too, that party control of the Senate has shifted in recent years more often than in the House. Since the 1980 election, the Senate has changed hands six times (and the House only twice). Democrats have held the Senate majority since 2007.

As for which side has the fundraising edge, it so far looks to be a draw. Republican Senate candidates have raised more money, but they've also spent more during the primary season. That leaves the average GOP Senate candidate with a bit of a disadvantage heading into the fall: about $800,000 cash on hand compared with the average Democratic candidate's $1.15 million, according to the Center for Responsive Politics. Here's a primer on the 2010 Senate races.

Thirty-seven of the Senate's 100 seats are up for vote on Nov. 2. Nineteen of those are currently held by Democrats and 18 by Republicans.

How many seats do Republicans need to win to take control of the Senate, and what are their odds of doing so?

The Republicans need to net 10 seats to take control of the Senate. The odds of doing so are slim, says Jennifer Duffy, senior editor for the Cook Political Report, but the GOP is likely to gain five to seven seats.

Even if Democrats keep control, they probably won't have anywhere close to the 60 votes needed to stop a filibuster and move legislation to the floor. The job of leading the Senate will get harder, no matter what the outcome in November.

How many seats are open?

Fourteen seats are open, meaning no incumbent is running. Three of those are unlikely to change party hands. One – North Dakota – is sure to switch from Democratic to Republican. Of the remaining 10, Democrats now hold five: Connecticut, Illinois, Pennsylvania, Indiana, and Delaware. Those states represent excellent pickup opportunities for Republicans. Republicans hold the other five – Florida, Kentucky, Missouri, New Hampshire, and Ohio – and they are fighting hard to hold onto those as well.

Arkansas:Sen. Blanche Lincoln (D) is lagging Republican challenger Rep. John Boozman by about 25 points. Ms. Lincoln surprised many by fending off a tough primary challenge, and her campaign war chest contained $1.9 million as of June 30, compared with Mr. Boozman's $484,000. But she's a Democrat in a traditionally red state, and her vote in favor of health care and her party's big spending aren't helping her now.

California:Sen. Barbara Boxer (D) has seen challenger Carly Fiorina, former Hewlett-Packard CEO, make steady gains in recent weeks. The senator's likability and job-approval ratings have fallen this year. With California's jobless rate stuck above 12 percent, many voters may be disappointed in the results from the huge federal stimulus and ready for a different approach.

Colorado:Sen. Michael Bennet (D) appears to have some ground to close against the GOP's Ken Buck, Weld County district attorney, who has been ahead in the polls. Senator Bennet, who was appointed to his seat last year, is portraying himself as a Washington outsider, given all the anti-incumbent sentiment among voters in this election cycle.

Nevada: Majority leader Harry Reid (D) is now in a position to compete for a seat he had been all but certain to lose. With the nomination of "tea party"-backed Sharron Angle, who has staked out some far-right positions, he has pulled to a virtual tie in a recent poll. Still, Nevada's dismal economy and Reid's record as a Washington insider appear to be hurting his standing with voters.

Washington: Three-term incumbent Sen. Patty Murray (D) holds a slim three-point lead over Republican challenger Dino Rossi, a former state senator, in recent polls. Mr. Rossi is using Murray's Senate Democratic caucus leadership role to suggest that she is a Washington insider who is committed to big spending.

Wisconsin: The primaries are not until Sept. 14, but polls show Sen. Russ Feingold (D) in a virtual tie with his likely Republican challenger, Ron Johnson, a self-funded candidate and plastics manufacturer. As much as Mr. Johnson, Senator Feingold's foe is the prevailing anti-incumbent climate.

Why does it matter which party has majority control of the Senate?

The party with the majority control of the Senate sets the chamber's agenda, putting its members in a powerful position to determine if and when legislation will be brought up. It can also control the Senate committees by appointing the chairs from its own members and by having a majority vote in each committee. Moreover, the majority party can decide whether to pursue congressional investigations into the administration's activities. Increasingly, the Senate requires 60 votes – the magic number to avoid a filibuster – to get major legislation through the chamber. So even if the Democrats keep the majority, the farther they move from 60, the harder it will be for them to pass anything big or controversial.

2010 Senate races: Did you know...?

... AN UNPRECEDENTED 15 WOMEN won (or are expected to win) their parties’ Senate nomination. Ten are Democrats; five are Republicans. Republican female senators are a rather rare breed – currently there are four (and one of those is retiring).
... TWO INCUMBENTS LOST in the primaries, which is rare. They are Sens. Arlen Specter (D) of Pennsylvania and Robert Bennett (R) of Utah. A third, Lisa Murkowski (R) of Alaska, is trailing a primary challenger with most (but not all) of the votes counted.... FLORIDA WILL HAVE A THREE-WAY race. Gov. Charlie Crist, formerly a Republican, is running as an independent against likely GOP standard-bearer Marco Rubio and the winner of the Democratic primary on Aug. 24.